CMOS image sensors are used to transform incident light energy into electrical signals, and are widely used in devices such as digital cameras and video recorders. FIG. 1 is a circuit diagram of a typical photocell of a CMOS image sensor. The photocell includes a photodiode PD that generates carriers responsive to incident light Lin. Referring to FIGS. 1 and 2, in an image capture cycle, carriers are transferred from the photodiode PD to a floating diffusion node FDN by a transfer transistor TTr responsive to a signal S_TG applied to a gate TG of the transfer transistor TTr. A potential developed at the floating diffusion node FDN is proportional to the amount of transferred change, and is used to drive the gate of an amplifier transistor FTr to control a current passing through a select transistor STr. A reset transistor RTr is used to reset the floating diffusion node FDN for a succeeding image capture cycle.
FIG. 3 is a cross-sectional view of a conventional CMOS photocell. A photodiode PD includes an n-type region 20 formed in a p-type substrate 10 and a p+-type 30 region formed on the n-type region 20. A transfer transistor 40 that couples the photodiode PD to an n+-type floating diffusion node region FDN in the substrate 10 includes a transfer gate TG overlying a channel region CH_T in the substrate 10. A reset transistor 50 that couples the floating diffusion node region FDN to a reset node region RN in the substrate 10 includes a reset gate RG overlying a channel region CH_R in the substrate 10. Light Lin is incident on the photodiode PD from a front side of the substrate 10 in an area F1 overlying the p+-type region 30. FIG. 4 illustrates a potential profile for the device shown in FIG. 3.